Many industrial operations are confronted by the build up of static charge on work pieces which then contribute to undesirable particulate contamination, unwanted movement, or other undesirable physical parameters associated with the work pieces. In the preparation of continuous films of sheet plastic materials, extended lengths of non-conductive plastic films pass rapidly over one or more rollers and accumulate substantial electrostatic charge that then attracts surface contaminants, and inhibits tight compaction in take-up rolls, impedes surface coating processes, and otherwise interferes with safe processing of the films.
Air ionizers, designed in a shape of a rod or a bar, are commonly positioned in close proximity to such moving webs to supply positive and negative ions for substantially neutralizing static charge on the web material. These air ionizers commonly contain pointed ionizing electrodes and operate at voltages of several kilovolts supplied to the ionizer via cables from remote generators positioned away from the ionizer. In large industrial applications, such webs may be several feet wide, operate at high linear speeds, and exhibit wide variations in the amount of static charge requiring neutralization at any given time or location along the moving web.
Typically, ionizing currents of about 1 to 5 microamperes per linear inch of the moving web are required for neutralization. The webs may vary in widths from several inches to 20 feet. This requires that the generators which supply such ionizers be capable of sustaining the output current of about 1-5 milliamperes at voltage levels of about 3-15 kilovolts.
There is a common problem with all air ionizer. This problem is dirt and residue accumulation on the tips of ionizing electrodes that limits their ionizing efficiency.
A problem with conventional ionizers that there is no economical and practical way to measure and monitor the ionizing efficiency of the electrodes without employing complex sensors and circuitry. For air ionizers with generators that produce high voltage output of the alternating current power at the power line frequency(AC) the difficulty of measuring the ionizing efficiency arises from the fact that the alternating potential applied to the electrodes couples capacitively to the electrically grounded components of the ionizer and the generator to produce a significant capacitive current that has a different phase and can substantially exceed the ionizing current.
For instance, in U.S. Pat. No. 5,017,876 the monitoring of the ion current from discharge electrodes of an AC ionizer is accomplished with a use of one or more sensors adjacently spaced from discharge electrodes. In one example of that device, one sensor picks up a capacitive current signal, while a second sensor picks up the total signal which represents the sum of the capacitive and corona (ion) currents. The outputs of the sensors are coupled to electronic circuitry, such as differential amplifier, to separate capacitive current from the total current signal. The problem with this approach, is that it requires adding sensors to the ionizer's construction. That increases the cost and manufacturing complexity of the equipment.
European Patent Application No. 97116167.4 (EP 0 844 726 A2) describes a different approach to detection of contamination on the discharge electrodes of an AC ionizer. In this application a complex electronic circuit with a microprocessor is employed to monitor and process a signal representing the output current of a high voltage AC transformer.
In another European Patent Application No. 97112236.1 (EP 0 850 759 A1) describes a system which includes an ionizer bar and circuitry for detection of contamination on ionizer electrodes. In order to achieve that the ionizer bar contains, in addition to ionizer electrodes, multiple contamination detecting sensors imbedded into the bar's body. That increases the cost and manufacturing complexity of the equipment.